The present invention relates to a process and apparatus for measuring forward and rearward three-dimensional curves and thickness of a corrective eyeglass lens while contour-grinding the circumference of a corrective lens and for using the measured values to grind a bevel or groove by means of a computer-controlled corrective lens edge grinding machine.
Described in German patent specification 38 42 601, held by the present applicant, and corresponding to U.S. Pat. No. 4,964,239, is a computer-controlled corrective lens edge grinding machine featuring shaft halves holding the corrective lens and an edge grinding disk which shapes the circumference of the lens along with a tracing device near the circumference of the corrective lens to measure the three-dimensional curves and the thickness of the lens, this tracing device being connected to a computer. The tracing device has arms situated in a fork-like arrangement one with respect to another and parallel to the plane of the grinding disk and positioned at a distance one to the other corresponding approximately to the width of the grinding disk. The shaft halves holding the corrective lens or the grinding disk and the tracing device execute reciprocating movements which exhibit either a constant amplitude with a minimum value corresponding to the distance between the arms or reciprocating movements the amplitude of which is determined by the contact of the corrective lens with the arm in each case, whereby the path covered is measured either directly or by way of the time consumed by the movement of the corrective lens or the grinding disk to and fro between a fixed reference plane and the reversing points for the reciprocal movement.
In this corrective lens contour-grinding machine, the corrective lens held between the shaft halves rotates continuously or stepwise during the grinding process, continuing even when the corrective lens is in the area near the edge of the grinding disk and in contact with one of the arms of the tracing device. If the pre-ground blank which already exhibits approximately the ultimate contour at the circumference exhibits severe concavity, which can easily be the case when dealing with very thick lenses and certain eyeglass frame shapes, then the measured values will be inaccurate. This can result, on the one hand, from the fact that, due to the concavity, the point of contact between the circumference of the corrective lens and the tracing device is not at the point where the corrective lens is in contact with the grinding disk and, on the other hand, may result from the fact that in the course of the further rotation of the shaft halves with the corrective lens in contact with the tracing device, there is already a considerable change in the thickness and/or in the three-dimensional curve at the forward or rearward face of the corrective lens. The result is that the location of a bevel or groove applied after final grinding of the corrective lens contour using computer-controlled edge grinding with a suitably profiled grinding disk cannot be determined ideally so that high-quality corrective lenses cannot be manufactured which fit perfectly in the selected eyeglass frames and offer an aesthetically favorable appearance.